HAL Introduction

Table of Contents

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HAL stands for Hardware Abstraction Layer. At the highest
level, it is simply a way to allow a number of building blocks to be
loaded and interconnected to assemble a complex system. The Hardware
part is because HAL was originally designed to make it easier to
configure LinuxCNC for a wide variety of hardware devices. Many of the
building blocks are drivers for hardware devices. However, HAL can do
more than just configure hardware drivers.

1. HAL is based on traditional system design techniques

HAL is based on the same principles that are used to design hardware
circuits and systems, so it is useful to examine those principles
first.

Any system (including a CNC machine), consists of
interconnected components. For the CNC machine, those components might
be the main controller, servo amps or stepper drives, motors, encoders,
limit switches, pushbutton pendants, perhaps a VFD for the spindle
drive, a PLC to run a toolchanger, etc. The machine builder must
select, mount and wire these pieces together to make a complete system.

1.1. Part Selection

The machine builder does not need to worry about how each individual
part works. He treats them as black boxes. During the design stage, he
decides which parts he is going to use - steppers or servos, which
brand of servo amp, what kind of limit switches and how many, etc. The
integrator’s decisions about which specific components to use is based
on what that component does and the specifications supplied by the
manufacturer of the device. The size of a motor and the load it must
drive will affect the choice of amplifier needed to run it. The choice
of amplifier may affect the kinds of feedback needed by the amp and the
velocity or position signals that must be sent to the amp from a
control.

In the HAL world, the integrator must decide what HAL components are
needed. Usually every interface card will require a driver. Additional
components may be needed for software generation of step pulses, PLC
functionality, and a wide variety of other tasks.

1.2. Interconnection Design

The designer of a hardware system not only selects the parts, he also
decides how those parts will be interconnected. Each black box has
terminals, perhaps only two for a simple switch, or dozens for a servo
drive or PLC. They need to be wired together. The motors connect to the
servo amps, the limit switches connect to the controller, and so on. As
the machine builder works on the design, he creates a large wiring
diagram that shows how all the parts should be interconnected.

When using HAL, components are interconnected by signals. The designer
must decide which signals are needed, and what they should connect.

1.3. Implementation

Once the wiring diagram is complete it is time to build the machine.
The pieces need to be acquired and mounted, and then they are
interconnected according to the wiring diagram. In a physical system,
each interconnection is a piece of wire that needs to be cut and
connected to the appropriate terminals.

HAL provides a number of tools to help build a HAL system. Some of
the tools allow you to connect (or disconnect) a single wire. Other
tools allow you to save a complete list of all the parts, wires, and
other information about the system, so that it can be rebuilt with a
single command.

1.4. Testing

Very few machines work right the first time. While testing, the
builder may use a meter to see whether a limit switch is working or to
measure the DC voltage going to a servo motor. He may hook up an
oscilloscope to check the tuning of a drive, or to look for electrical
noise. He may find a problem that requires the wiring diagram to be
changed; perhaps a part needs to be connected differently or replaced
with something completely different.

HAL provides the software equivalents of a voltmeter, oscilloscope,
signal generator, and other tools needed for testing and tuning a
system. The same commands used to build the system can be used to make
changes as needed.

1.5. Summary

This document is aimed at people who already know how to do this kind
of hardware system integration, but who do not know how to connect the
hardware to LinuxCNC. See the Remote Start Example section in the HAL UI Examples documentation.

The traditional hardware design as described above ends at the edge of
the main control. Outside the control are a bunch of relatively simple
boxes, connected together to do whatever is needed. Inside, the control
is a big mystery — one huge black box that we hope works.

HAL extends this traditional hardware design method to the inside of
the big black box. It makes device drivers and even some internal parts
of the controller into smaller black boxes that can be interconnected
and even replaced just like the external hardware. It allows the
system wiring diagram to show part of the internal controller, rather
than just a big black box. And most importantly, it allows the
integrator to test and modify the controller using the same methods he
would use on the rest of the hardware.

Terms like motors, amps, and encoders are familiar to most machine
integrators. When we talk about using extra flexible eight conductor
shielded cable to connect an encoder to the servo input board in the
computer, the reader immediately understands what it is and is led to
the question, what kinds of connectors will I need to make up each
end. The same sort of thinking is essential for the HAL but the
specific train of thought may take a bit to get on track. Using HAL
words may seem a bit strange at first, but the concept of working from
one connection to the next is the same.

This idea of extending the wiring diagram to the inside of the
controller is what HAL is all about. If you are comfortable with the
idea of interconnecting hardware black boxes, you will probably have
little trouble using HAL to interconnect software black boxes.

2. HAL Concepts

This section is a glossary that defines key HAL terms but it is a bit
different than a traditional glossary because these terms are not
arranged in alphabetical order. They are arranged by their relationship
or flow in the HAL way of things.

Component

When we talked about hardware design, we referred
to the individual pieces as parts, building blocks, black boxes,
etc. The HAL equivalent is a component or HAL component. (This
document uses HAL component when there is likely to be confusion with
other kinds of components, but normally just uses component.) A HAL
component is a piece of software with well-defined inputs, outputs, and
behavior, that can be installed and interconnected as needed.

Parameter

Many hardware components have adjustments that
are not connected to any other components but still need to be
accessed. For example, servo amps often have trim pots to allow for
tuning adjustments, and test points where a meter or scope can be
attached to view the tuning results. HAL components also can have such
items, which are referred to as parameters. There are two types of
parameters: Input parameters are equivalent to trim pots - they are
values that can be adjusted by the user, and remain fixed once they are
set. Output parameters cannot be adjusted by the user - they are
equivalent to test points that allow internal signals to be monitored.

Pin

Hardware components have terminals which are used to
interconnect them. The HAL equivalent is a pin or HAL pin. (HAL
pin is used when needed to avoid confusion.) All HAL pins are named,
and the pin names are used when interconnecting them. HAL pins are
software entities that exist only inside the computer.

Physical_Pin

Many I/O devices have real physical pins or
terminals that connect to external hardware, for example the pins of a
parallel port connector. To avoid confusion, these are referred to as
physical pins. These are the things that stick out into the real
world.

Signal

In a physical machine, the terminals of real
hardware components are interconnected by wires. The HAL equivalent of
a wire is a signal or HAL signal. HAL signals connect HAL pins
together as required by the machine builder. HAL signals can be
disconnected and reconnected at will (even while the machine is
running).

Type

When using real hardware, you would not connect a 24
volt relay output to the +/-10V analog input of a servo amp. HAL pins
have the same restrictions, which are based upon their type. Both pins
and signals have types, and signals can only be connected to pins of
the same type. Currently there are 4 types, as follows:

bit - a single TRUE/FALSE or ON/OFF value

float - a 64 bit floating point value, with approximately 53 bits of
resolution and over 1000 bits of dynamic range.

u32 - a 32 bit unsigned integer, legal values are 0 to 4,294,967,295

s32 - a 32 bit signed integer, legal values are -2,147,483,647 to
+2,147,483,647

Function

Real hardware components tend to
act immediately on their inputs. For example, if the input voltage to a
servo amp changes, the output also changes automatically. However
software components cannot act automatically. Each component has
specific code that must be executed to do whatever that component is
supposed to do. In some cases, that code simply runs as part of the
component. However in most cases, especially in realtime components,
the code must run in a specific sequence and at specific intervals. For
example, inputs should be read before calculations are performed on the
input data, and outputs should not be written until the calculations
are done. In these cases, the code is made available to the system in
the form of one or more functions. Each function is a block of code
that performs a specific action. The system integrator can use
threads to schedule a series of functions to be executed in a
particular order and at specific time intervals.

Thread

A thread is a list of functions that
runs at specific intervals as part of a realtime task. When a thread is
first created, it has a specific time interval (period), but no
functions. Functions can be added to the thread, and will be executed
in order every time the thread runs.

As an example, suppose we have a parport component named hal_parport.
That component defines one or more HAL pins for each physical pin. The
pins are described in that component’s doc section: their names, how
each pin relates to the physical pin, are they inverted, can you change
polarity, etc. But that alone doesn’t get the data from the HAL pins to
the physical pins. It takes code to do that, and that is where
functions come into the picture. The parport component needs at least
two functions: one to read the physical input pins and update the HAL
pins, the other to take data from the HAL pins and write it to the
physical output pins. Both of these functions are part of the parport
driver.

3. HAL components

Each HAL component is a piece of software with well-defined inputs,
outputs, and behavior, that can be installed and interconnected as
needed. This section lists some of the available components and a brief
description of what each does. Complete details for each component are
available later in this document.

3.1. External Programs with HAL hooks

motion

A realtime module that accepts NML
[Neutral Message Language provides a mechanism for handling
multiple types of messages in the same buffer as well as simplifying
the interface for encoding and decoding buffers in neutral format and
the configuration mechanism.] motion commands and interacts with HAL

iocontrol

A user space module that accepts NML I/O commands and
interacts with HAL

classicladder

A PLC using HAL for all I/O

halui

A user space program that interacts with HAL and sends NML
commands, it is intended to work as a full User Interface using
external knobs & switches

3.2. Internal Components

stepgen

Software step pulse generator with position loop. See section [sec:stepgen]

3.4. Tools and Utilities

A full featured digital storage oscilloscope for HAL
signals. See the Halscope section.

Each of these building blocks is described in detail in later chapters.

4. Timing Issues In HAL

Unlike the physical wiring models between black boxes that we have
said that HAL is based upon, simply connecting two pins with a
hal-signal falls far short of the action of the physical case.

True relay logic consists of relays connected together, and when a
contact opens or closes, current flows (or stops) immediately. Other
coils may change state, etc, and it all just happens. But in PLC
style ladder logic, it doesn’t work that way. Usually in a single pass
through the ladder, each rung is evaluated in the order in which it
appears, and only once per pass. A perfect example is a single rung
ladder, with a NC contact in series with a coil. The contact and coil
belong to the same relay.

If this were a conventional relay, as soon as the coil is energized,
the contacts begin to open and de-energize it. That means the contacts
close again, etc, etc. The relay becomes a buzzer.

With a PLC, if the coil is OFF and the contact is closed when the PLC
begins to evaluate the rung, then when it finishes that pass, the coil
is ON. The fact that turning on the coil opens the contact feeding it
is ignored until the next pass. On the next pass, the PLC sees that the
contact is open, and de-energizes the coil. So the relay still switches
rapidly between on and off, but at a rate determined by how often the
PLC evaluates the rung.

In HAL, the function is the code that evaluates the rung(s). In fact,
the HAL-aware realtime version of ClassicLadder exports a function to
do exactly that. Meanwhile, a thread is the thing that runs the
function at specific time intervals. Just like you can choose to have a
PLC evaluate all its rungs every 10 ms, or every second, you can define
HAL threads with different periods.

What distinguishes one thread from another is not what the thread
does - that is determined by which functions are
connected to it. The real distinction is simply how often a thread
runs.

In LinuxCNC you might have a 50 us thread and a 1 ms thread.
These would be created based on BASE_PERIOD and SERVO_PERIOD, the
actual times depend on the values in your ini file.

The next step is to decide what each thread needs to do. Some of those
decisions are the same in (nearly) any LinuxCNC system—For instance,
motion-command-handler is always added to servo-thread.

Other connections would be made by the integrator. These might include
hooking the STG driver’s encoder read and DAC write functions to the
servo thread, or hooking stepgen’s function to the base-thread, along
with the parport function(s) to write the steps to the port.